ISSUE_TEMPLATE.md: Add mention of online formatter (pytest-dev#481)

People can try out https://black.now.sh/?version=master to test against master. That should make issue reporting easier.

See jpadilla/black-playground#6 (comment). Thanks @jpadilla!

.github/ISSUE_TEMPLATE.md

@@ -6,9 +6,11 @@ Python version:
 *Black* version:
 Does also happen on master:
 
-To answer the last question, follow these steps:
-* create a new virtualenv (make sure it's the same Python version);
-* clone this repository;
-* run `pip install -e .`;
-* make sure it's sane by running `python setup.py test`; and
-* run `black` like you did last time.
+To answer the last question, you have two options:
+1. Use the online formatter at https://black.now.sh/?version=master, which will use the latest master branch.
+2. Or run black on your machine:
+    * create a new virtualenv (make sure it's the same Python version);
+    * clone this repository;
+    * run `pip install -e .`;
+    * make sure it's sane by running `python setup.py test`; and
+    * run `black` like you did last time.